The invention relates to the separation of fine particulates from gas streams by means of electrostatic precipitators, and more particularly to the baffle assemblies used as distributors for the particulate-laden gas streams fed into the precipitators.
Electric precipitators are well known in the art for their ability to separate fine particulates present in gas streams An example of an electrostatic precipitator of the plate type is illustrated in U.S. Pat. No. 4,026,683 issued to Earle S. Snader et al. on May 31, 1977.
In these types of electrostatic precipitators, gas flows through a duct past a distributor plate, or baffle, and then proceeds in a substantially horizontal plane through the precipitator through broadly defined gas passages In these gas passages, a plurality of discharge wire electrodes and collector electrodes are suspended within the precipitator and contact the dust laden gas stream as it proceeds through the precipitator. The discharge wire electrodes ionize the particles in the gas stream flowing past them and the ionized particles are then attracted to and deposited on the vertical surfaces of the collector electrodes.
The collector electrodes are in the form of flat plates vertically suspended and in proximity to, but always separate from, the discharge wire electrodes The dust particles which cling to the vertical collector electrodes, are dislodged and fall when the collector electrodes are periodically rapped. The dust falls to the bottom of the precipitator where it is collected in hoppers suspended below the collector electrodes An opening at the base of the hopper provides an outlet for periodic removal of the separated particulates.
Such electrostatic precipitators have come into wide use because of their ability to separate extremely fine particulates, as low as one micron, and even less than one micron, from the gas stream efficiently and quickly and without any excessive pressure drop in the gas stream being passed through the precipitator.
The plate-type electrostatic precipitator described above is appropriate for many applications; however, when the gas stream contains a high dust level, for example, as from electric furnaces producing phosphorus, the electrostatic precipitator that we would prefer to employ is a vertical tube electrostatic precipitator. This contains a plurality of vertically oriented tubes which act as the vertical collector electrodes. Within each of these tubes, a wire electrode is suspended along the center line which acts as the discharged electrode.
This configuration is preferred where high dust levels are found in the gas stream because the plurality of small tubes present uniform collecting surfaces and the electrical field is uniform within each of the tubes. That is, the distance between discharge wire electrodes and the internal surface of the tube which serves as the collecting electrode, is uniform throughout the length of the tube. Also, since this distance between the wire and tube electrodes is relatively small it permits more efficient collection of dust particles. The distribution of dust along the collecting electrode, namely, the interior surface of the collecting tube, is generally more uniform than in plate-type electrodes because of the uniformity of distance between the wire electrodes and the corresponding collecting electrodes.
In the treatment of dust-laden gas streams from an electric furnace used to produce elemental phosphorus, it has long been a problem that the electrostatic precipitators, whose function it is to remove dust from the elemental phosphorus and carbon monoxide gas stream exiting the electric furnace, have suffered from low collection efficiency. This is due principally to the poor gas distribution in the electrostatic precipitator. The gas enters on one side of the precipitator, sweeps to the opposite, and is thus unevenly distributed with the vast majority flow being on the far side of the gas inlet. Thus, the far side tubes suffer excessive gas velocities and consequently do not collect high amounts of dust, while the near side tubes see only a small flow and also, therefore, collect a small fraction of the dust. This poor dust collection performance is a major contributor to sludge production in the resulting condensed elemental phosphorus. The unremoved particles of dust combine with the elemental phosphorus to form a nonwettable globule that does not readily separate into phosphorus and water layers. This results in an intermediate sludge layer between the water and phosphorus layers that is relatively stable and which makes recovery of pure phosphorus difficult. Reduction of the amount of sludge produced is an important task facing the manufacture of elemental phosphorus by the electric furnace method. Thus, the design and operation of a more efficient electrostatic precipitator would directly impact on decreasing the significant sludge problem.
In operation, the particulate-containing gas stream is introduced into the electrostatic precipitator at its base and the gas stream is allowed to flow upwardly through the plurality of vertical tubes. The dust particles are ionized by the discharge wire electrodes suspended within the tubes and collect on the inside surfaces of the tubes which serve as the collecting electrode. Periodically, these tubes are rapped and the collected dust on the inside surfaces of the tubes is broken free and falls through the tubes into a hopper at the base of the electrostatic precipitator where a conveying screw removes it from the precipitator.
One of the problems in the operation of a vertical tube electrostatic precipitator is that the gas which is introduced at the base of the precipitator is difficult to distribute uniformly through the precipitator. This is because the gas inlet is usually at an angle of at least 90.degree. from vertical, and usually much more than 90.degree., when it enters the base of the electrostatic precipitator This means that the gas must make at least a 90.degree. turn before it begins its upward ascent through the tubes of the electrostatic precipitator.
In the absence of a distributor plate, the flow of the gas tends to accumulate on one side of the electrostatic precipitator, as described above, generally the side opposite the gas inlet. Once the gases enter predominately those tubes on one side of the precipitator they must continue to flow through the tubes entered. They can no longer be uniformly distributed throughout the precipitator, since, once entered, the tubes define the flow path throughout the entire precipitator.
The presence of a conventional distributor plate (a plate with multiple holes punched through it) would, of course, result in a more uniform distribution of gas into the base of the electrostatic precipitator; however, such a plate prevents proper removal of the collected dust which must of necessity pass through the distributor plate in order to fall downwardly into the hopper and be removed from the electrostatic precipitator. Also, use of such a distributor plate generally results in rapid fouling of the plate even after only a very short time of operation. For this reason, such distributor plates are not compatible with this kind of apparatus.
The present invention has for its objective a process and means for providing an electrostatic precipitator that obviates the problems aforesaid.